1. Field of the Invention
The present invention relates to products and processes that use fly-ash for producing aggregates useful in the production of structural products.
2. Discussion of the Background Art
Aggregate, as referred to herein, is a term which includes natural aggregate, such as sand, gravel and crushed stone, as well as synthetic aggregate of various kinds, including light-weight aggregate such as cinders and aggregate produced through the combustion of pulverized powdered coal. Generally, aggregate may be used in a variety of construction and landscaping applications. For example, aggregate is an essential ingredient in most concrete formulations, including, both pre-cast and cast-in-place concrete. Aggregate is also used as a foundation material for paving and as a backfill material. Aggregate is also used in landscaping applications in both a decorative and functional manner.
When used as a construction material, aggregate is frequently required to meet certain requirements relating to its physical and chemical characteristics. Those characteristics indicate the level of quality to which it conforms. Durability is one of the most significant elements of quality in aggregate. The prior art recognizes there is a great desire to use fly-ash as a major ingredient in the manufacture of a synthetic aggregate. Fly-ash is the finely divided residue resulting from the combustion of material containing coal as its major fuel source for combustion, such as that which is produced in electricity generating power plants. The fuel may also contain tires, wood, and/or other petro-coke constituents. It is typically collected as a fine particulate from the combustion gases before they are discharged into the atmosphere. Generally, there are two more particular classes of fly-ash, the composition of which is dependent upon the composition of the original combustible material from which it is derived. Class F fly-ash is normally produced from anthracite or bituminous coal and has pozzolanic properties, i.e. little or no cementitious value, but capable of chemically reacting with calcium hydroxide to form compounds possessing cementitious properties. Class C fly-ash is normally produced from lignite or sub-bituminous coal and has some cementitious properties in addition to pozzolanic properties.
Class C fly-ash is known to be highly reactive when mixed with water and flash setting (setting which occurs in about 2 minutes or less) will generally occur unless a reaction sequestering admixture is utilized. Use of Class C fly-ash in the manufacture of light-weight aggregate and light-weight cement formulations has been suggested, for example, in U.S. Pat. Nos. 4,624,711, and 4,741,782, both to Styron, and U.S. Pat. No. 4,659,385 to Costopoluos et al. Styron suggests the use of Class C fly-ash along with a foaming agent and an accelerator, expenses the present invention seeks to avoid, to make light-weight aggregate in a pelletizing process. Depending upon the type of fly-ash used, Styron also recommends the addition of Portland cement, another expense the present invention seeks to avoid. Costopoluos suggests the use of Class C fly-ash to make a light-weight building material and the use of air entraining and chemical foaming agents which is another expense the present invention seeks to avoid.
The many methods and processes disclosed in the prior art have costs associated with them and the present invention is directed specifically towards reducing the cost of manufacturing such aggregates. Among the many costly apparatus and procedures which the present invention seeks to eliminate are pellatizers, heated kilns, large storage silos, pellatizering, vibration and other types of machine compaction, and transporting slurry used to make the aggregate.
U.S. Pat. No. 5,484,479 entitled "Method of Manufacturing Synthetic Aggregate" describes much of the background of aggregates and their use as well as disclosing a method for making synthetic aggregate using fly-ash. The method proscribes a method which combines fly-ash with water to form a soil-like mixture which is then compacted into slabs and allowed to set. After setting, the material is crushed to appropriate size. Since fly-ash and water mixtures set quickly, and since compaction should take place prior to setting of the mixture, a set sequestering admixture may be used to facilitate the process. This process uses dry Class C fly-ash metered from a container which is then introduced to a mixing device where it is sprayed with a fine mist of water droplets. The fly-ash and water are metered at rates sufficient to produce a soil-like mixture. The mixture is then distributed and uniformly consolidated, preferably by mechanical compaction, to densify the mixture. The densified mixture is then allowed to cure to a hardened mass and after a predetermined minimum time, the material is then broken or crushed into appropriately sized aggregate conforming to specific gradations established for conventional crushed aggregates. However, expensive materials are used to promote the formulation of stable ettringite and straetlingite compounds which is accomplished with the use of an admixture like those used to retard the setting of portland cement. The compaction is also a costly step which the present inventions seeks to avoid. The patent suggests using sugar to form the admixture and that lignosites, both calcium and sodium, are also usable to effectively sequester or retard the setting process.
The present invention provides a method used in the process for producing an aggregate that is economical and consistent with strength standards as detailed by ASTM's (American Society for Testing Materials) specifications for aggregate. One purpose of the present invention is designed to produce such an aggregate at a low cost without the use of additional fuel for heating, heaters, or pelletizing equipment. To this end, the preferred embodiment of the present invention makes use of a radial stacker and a preferred mixture of fly-ash, hydrated lime, and alkali hydroxide. Radial stackers are well known devices as exemplified by those disclosed in U.S. Pat. Nos. 5,609,836 and 4,172,518.
U.S. Pat. No. 4,04,147 discloses the use of a radial stacker to form conical piles of a mixture containing a heavy metal immobilizing agent such as either Portland cement, lime, gypsum, caustic soda, magnesium oxide or sodium hydroxide and a volumetric stabilizing agent such as either Portland cement or sodium silicate. However the mixture and process are such that only partial hydration occurs. The resulting mixture is retained for only sufficient enough time to allow the immobilizing and stabilizing reactions to occur and weak agglomerations to form. This process is more expensive and time consuming than the present invention and does not produce as hard an aggregate. The preferred mixture of the present invention was disclosed in U.S. Pat. No. 5,704,972 entitled "Product and Process for Strong Light-Weight Aggregate".